12 April 2016

odd CRC error (in a good signal)

Some days ago, just to spend some time, I was listening to  loud and clear HFDL messages exchanged between the ARINC ground station of Al Muharraq and some aircrafts. No problems for decoding all the messages but one of these failed due to a CRC error. Fortunately I was recording the IQ signal so I decided to undertand the reason of that CRC error, since the received signals were very good indeed.
Once isolated and demodulated the error burst I opened the resulting bitstream in order to verify the HFDL Physical layer protocol (PPDU) structure and it was ok. As expected: 25ms frame length corresponding to a 45 symbols frame for the data-segment, thirty of which are information-carrying (user data) symbols and fifteen are known 2-PSK probe symbols. I also detected the "T" string (000 100 110 101 111 repeated 9 times) closing the  preamble-segment (pics 1,2).

pic. 1 - PPDU structure
pic. 2 - bistream of the error burst after demodulation
From a certain point onwards the bistream shows a repeated regular pattern in the data field (pic. 3). The length of this repeated pattern is 120 bit: just as the legth of the PPDU scrambler (pic. 4). This could mean that there is no phase changes in the user-data or no data at input and in this situation the FEC decoder reveals errors in CRC computing.

Pic. 3
Pic. 4

No comments:

Post a Comment